Digital terrestrial television signals suffer from multi-path interference which leads to pre-echoes which are caused by signals received via paths which are shorter than the path travelled by the principal signal and post-echoes which are caused by signals that have travelled along longer paths. A digital television receiver therefore comprises circuitry, including an adaptive equaliser, to suppress the echoes.
A common method of equalisation of such signals is to use a combination of a feed-forward equaliser (FFE) and a decision-feedback equaliser (DFE). These two different types of equaliser have different characteristics and can be used to deal with different aspects of the interference. The FFE can deal with both precursor and postcursor intersymbol interference (ISI), e.g. both pre-echoes and post-echoes, whilst the DFE can only deal with postcursor ISI but, unlike the FFE, does not enhance noise.
In order to use this combination of a FFE and DFE, initial equaliser coefficients are calculated; however, these initial coefficients are not likely to provide perfect equalisation results straight away and consequently the coefficients may be updated during operation of the equaliser. This updating of coefficients also enables the equaliser to adapt to possible channel changes. Typically the least-mean-square algorithm is used to update coefficients.
The embodiments described below are not limited to implementations which solve any or all of the disadvantages of known methods of updating equaliser coefficients.